Loop from A to Z in jQuery

Use char codes: JavaScript Char Codes

for (var i = 65; i <= 90; i++) {
    $('#select_id_or_class').append('<option>' + String.fromCharCode(i) + '</option>');
}

You can do this with the following

    var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("");
    $.each(alphabet, function(letter) {
        $('.example-select-menu').append($('<option>' + alphabet[letter] + '</option>'));
    });

This answer demonstrates the nice idea that you do not need a hardcoded string. In short:

for (i = 65; i <= 90; i++) { arr[i-65] = String.fromCharCode(i).toLowerCase(); }